The invention relates to brush wear indicator assemblies for use with dynamoelectric machines to sense a predetermined degree of brush wear and signal the occurrence of such wear to an operator. More particularly, the invention relates to a contact and terminal assembly for a brush wear indicator that is typically operated in environments that subject the assembly to severe mechanical stresses and relatively high temperatures.
In many applications of dynamoelectric machines utilizing carbon brushes it is desirable to provide some means for indicating when the brushes on the machines become sufficiently worn to warrant their replacement before they either fail to adequately conduct current or before embedded conductors in the brushes are forced into contact with the slip-ring or commutator on which the brushes ride. A considerable variety of such brush wear indicator mechanisms are described in my co-pending U.S. patent application Ser. No. 183,904, which was filed Sept. 4, 1980, and issued as U.S. Pat. No. 4,329,683. Another copending U.S. patent application, Ser. No. 183,920, also filed Sept. 4, 1980, and issued as U.S. Pat. No. 4,348,608, discusses several alternative types of prior art brush wear indicator devices. In addition, this latter patent discloses a brush wear indicator in which a pre-stressed spring of volute construction is used to actuate a brush wear indicating signal circuit by moving a coiled portion of the spring into engagement with a relatively fixed electrical terminal of a signal circuit, thereby to energize the signal circuit with current flowing through the spring into the fixed terminal. Thus, as shown by that patent, it is already known in the brush wear indicator field that a brush biasing spring can be suitably employed as an electrical contactor for energizing a signal circuit. Moreover, it has been recognized that a terminal of a brush wear indicator circuit can be mounted in fixed relationship on, or immediately adjacent to, a fixed end of such a spring, thereby to position the terminal in the path of movement of the coiled portion of the spring.
It has also been found that the type of rigid terminal assemblies and associated bonding and insulating materials and processes used in making the kind of structure described in the patent last mentioned above, often result in the brush-follower spring constant being undesirably varied. Specifically, such rigid terminal members impede the tendency of the spring to bend or coil, and if high temperature processes, such as soldering or brazing are used to bond the terminal to the brush-follower spring, the heat of the process relaxes the spring. Such undesirable variations in the spring constant limit the spectrum of applications for which rigid terminal assemblies are suitable; consequently, a recognized desirable objective is to provide an improved terminal and contact assembly that will make more feasible to use a brush-biasing spring as a circuit-closing contactor for a brush wear indicating circuit.
Another desirable improvement for such a contact and terminal assembly for brush wear indicators would be to provide some suitable means for assuring positive engagement of a brush wear signal circuit contactor at a precisely determinable point in the path of movement of the coiled portion of a brush biasing spring as it winds up. The advantage of being able to so precisely determine a given point in the movement of the brush-biasing spring is that it then becomes possible to accurately detect and signal a preselected degree of wear of a brush that is biased against a running surface by the spring.
In addition to overcoming the problem of inducing changes in the spring constant by mounting a terminal and associated insulating member on a brush-biasing spring, it is necessary in the manufacture of contact and terminal assemblies for such brush wear indicators to employ methods and materials that are suitable for the relatively high temperatures, and mechanical stresses that are often encounted in typical dynamoelectric machine brush applications. In particular, where adhesive materials are used in the manufacturing processes, it is necessary to preserve desired characteristics such as reliable intercomponent bonding and long-lasting flexibility of parts of the assembly, in order to avoid fatigue failures during the anticipated long life of such assemblies. Moreover, reliability of the electrical signal indicating and conducting components of brush wear contact and terminal assemblies is important due to the nature of the warning function required from such apparatus.
It has been found that the reliability of contact and terminal assemblies for brush wear indicators can be significantly improved by practicing certain novel method steps in their manufacture. In the present invention, such method steps are selected and arranged to result in the manufacture of assemblies that are capable of performing in relatively high temperature environments without loss of any of the desired characteristic features of the components of the assembly and without undue distortion of the spring constant of an associated brush biasing spring on which the assembly is applied.